(Technical Field)
The present invention relates to an electromagnet for bending an orbit of a charged particle beam and a magnetic field generating apparatus for generating a magnetic field in an air gap and, in particularly to an electromagnet suitable for use in a physical experimentation use accelerator, an industrial use accelerator and a medical treatment use accelerator or a magnetic field generating apparatus for use in a medical treatment use diagnosing instrument such as MRI (Magnetic Resonance Imager).
(Prior Arts)
In generally, an electromagnet used in an accelerator is designed to have a desirable magnetic field distribution in a predetermined area which is determined by a size of a charged particle beam (hereinafter, it is called as a beam). However, in a case when a magnetic field strength increases, due to an affect of a magnetic saturation of an iron core which is used in a magnetic pole of the electromagnet, a leakage magnetic field from the magnetic pole increases. According to the affect of this leakage magnetic field, since an area (hereinafter, it is called as a preferable magnetic filed area) becomes narrow, where an amount of a displacement from a desirable magnetic field distribution is less than a predetermined value, in the electromagnet a maximum magnetic field strength enable to use actually is limited.
For example, in a case of a bending electromagnet for use in an accelerator, when a radius of curvature and a maximum magnetic field strength are determined, a maximum energy of the beam obtained by the accelerator is determined. As a result, in a case where there is a chance of a use of the beam with a wide energy range, since the maximum magnetic field strength of the electromagnet is limited by the above stated reasons, it is necessary to employ a bending electromagnet having a large radius of curvature.
As to the prior art relating to the bending electromagnet, a technique about a bending electromagnet is disclosed in Japanese patent laid-open publication No. Hei 5-47,547 (hereinafter it is called as a first prior art). In such a bending electromagnet, a projection which is called as a magnetic pole shim is provided at an end portion of magnetic poles which are arranged oppositely.
Further, recently it has paid to attention to a medical treatment use diagnosing instrument using a magnetic field such as an open type MRI, in which a permanent magnet having a flexibility is utilized, and also it has studied about a method for generating a high magnetic field and a good uniformity magnetic field in an air gap.
In the magnetic field generating apparatus having the permanent magnet, a conventional technique for generating the magnetic field having the high magnetic field and the good uniformity magnetic field is disclosed Japanese patent laid-open publication No. Hei 5-243,037 (hereinafter, it is called as a second prior art). This magnetic field generating apparatus comprises pole pieces which are arranged oppositely and at an end portion a magnetic pole shim is provided, and permanent magnets.
Further, in this magnetic field apparatus, it is disclosed about a central portion of the pole piece which is moved toward an upper portion and a lower portion. A lateral cross-section of an upper portion magnetic pole portion in this magnetic field generating apparatus is shown in FIG. 12. In FIG. 12, 101 denotes a permanent magnet, 103 denotes a yoke, 108 denotes a permanent magnet fixing use bolt, and 121 and 122 denote pole pieces.
In the above stated first prior art, by concentrating a line of magnetic force which spreads from an end portion of the magnetic pole toward a lateral direction, it aims to spread a good field region (stated in a latter portion) formed between the magnetic poles.
However, since it is necessary to provide the projection to the magnetic pole, a magnetic pole structure becomes a complicated one. Further, since the magnetic field distribution formed between the magnetic poles is affected largely by a shape of the projection, it requires a high processing accuracy for manufacturing the magnetic poles. In company with this, also a manufacturing cost becomes high.
Further, in the above stated second prior art, similarly to the first prior art, since it is necessary to provide the projection to the pole piece, the pole piece structure becomes a complicated one. Further, since the magnetic field distribution formed between the pole pieces is affected largely by a shape of the projection, it is required a high processing accuracy for the pole piece.
Further, in the second prior art, since a movement mechanism is provided by dividing into the pole piece, it requires a large mechanical force to a mechanical sliding portion, a high reliability, and a high reproducibility. In company with those requirements, also a manufacturing cost becomes high.
On the other hand, in the defecting electromagnet in which a projection is not provided to a magnetic pole, since a maximum magnetic field strength is limited according to an affect of a magnetic saturation, in a case where it aims to obtain a wide range beam energy, the bending electromagnet becomes a large size one and also the accelerator becomes a large size one. Similarly to, the magnetic field generating apparatus in which the projection is not provided at the pole piece becomes a large size one.